As industries of semiconductors, ships, and industrial parts are developed, a market for abrasives is expanding. According to the spread of products requiring precision processing, abrasives need precision and reliability.
In addition, there is a demand for developing a highly-functional, abrasive grain for polishing a product manufactured of stainless steel, titanium, a nickel alloy or aluminum. To develop the highly-functional abrasive grain, extension of a life span, enhancement of durability, and enhancement of a processing degree of the abrasive are required.
Alumina (Al2O3) is widely used as an abrasive, since it has excellent physical properties such as excellent heat resistance and corrosion resistance and a high hardness compared to various ceramic elements.
Since an alumina abrasive has an intermediate characteristic between an artificial diamond or CBN and melted alumina, a new market for replacing part of the demand for an artificial diamond or CBN and melted alumina is being created.
However, a conventional abrasive using an alumina grain has many difficulties in enhancing durability and a cutting rate. During polishing or grinding, since grain particles are broken and thus the grain is released, it is impossible to have contact between a grinded surface and the grain, which becomes a cause of reducing a polishing or grinding rate.
Accordingly, there is a demand for developing an alumina abrasive which is highly dense and has a high hardness.
The present disclosure is directed to providing a method of manufacturing an alumina-based abrasive grain which is highly dense, has a high hardness and a high purity, has a high reproducibility and is capable of being produced on a large scale using boehmite powder and activated alumina powder, and an alumina-based abrasive grain manufactured thereby.
One aspect of the present invention provides a method of manufacturing an alumina-based abrasive grain, which includes (a) preparing boehmite powder and activated alumina powder as starting materials, (b) forming a sol by wet-blending and crushing the boehmite powder, the activated alumina powder, a solvent and a deflocculant, (c) heating the sol at a first temperature which is higher than a room temperature and lower than a boiling point of the solvent and stirring the sol so as not to generate a precipitate, (d) forming a gel by heating the sol at a second temperature higher than the first temperature at which a viscosity of the sol is increased and the sol becomes a paste, (e) blending the gel with an organic solvent and performing wet crushing on the resulting mixture, (f) preparing a powder by drying the wet-crushed gel, (g) blending a binder and a solvent with the dried product, which is the powder, and molding the resulting mixture, (Ii) calcining the molded product, (i) performing dry crushing on the calcined product, and (j) sintering the dry-crushed product to transform activated alumina and boehmite contained therein to an α-Al2O3 crystal phase.
The activated alumina powder may be at least one transition alumina selected from γ-Al2O3, δ-Al2O3, θ-Al2O3 and η-Al2O3.
The boehmite powder and the activated alumina powder may be blended in a weight ratio of 1:1 to 3:1.
The deflocculant may be at least one acid selected from nitric acid (HNO3) and hydrochloric acid (HCl), and the sol may have a pH of 2 to 5.
The wet-blending may be performed by wet ball milling, an inner wall of a ball mill for the wet ball milling may be lined with alumina, and a ball used in the wet ball milling may be formed of alumina.
The heating in the step (c) may be performed by injecting a hot air whose temperature is higher than the room temperature and lower than the boiling point of the solvent.
The injection of the hot air may be performed by supplying an air to a heat exchanger using a pump to heat the air, and injecting the hot air into a heating agitator. As the hot air is introduced under the sol, thereby inducing convection of the sol, generation of a precipitation may be inhibited, and uniform blending may be performed.
The gel and the organic solvent may be blended in a weight ratio of 1:0.1 to 3, and as the organic solvent, at least one alcohol selected from ethanol and methanol may be used.
The wet crushing may be performed by wet ball milling, an inner wall of a ball mill for the wet ball milling may be lined with alumina, and a ball used in the wet ball milling may be formed of alumina.
The dry crushing may be performed by dry ball milling, an inner wall of a ball mill for the dry ball milling may be lined with alumina, and a ball used in the dry ball milling may be formed of alumina.
The molding may be continuous compression molding, which may include supplying the dried product through a hopper of a compression molding machine, transferring the dried product injected through the hopper into a cylinder at which an end of a screw is placed while providing compounding and compression due to rotation of the screw, transferring the resulting product transferred into the cylinder to a segment roll by the rotation of the screw, transferring the resulting product transferred to the segment roll to an outlet with compression in-between the segment rolls, and molding the resulting product transferred to the outlet in a pellet type after discharging to an outside.
The binder may be at least one selected from polyvinylalcohol, polyethyleneglycol and cellulose, and blended at 0.1 to 30 parts by weight with respect to 100 parts by weight of the dried product.
The calcination may be performed at 400 to 800° C., which is higher than a temperature for burning off the binder, in an oxygen atmosphere.
The calcination may be performed by heating a rotary tube furnace at a calcination temperature of 400 to 800° C. using a gas burner, charging the molded product to the rotary tube furnace using a feeder for fuel supply, performing calcination while the charged product flows toward the gas burner due to the rotation of a tilted rotary tube furnace, and transferring the calcined product to a tilted rotary cooler into which an external cold air flows for cooling.
The sintering may be performed at 1200 to 1650° C., which is higher than the temperature at which the activated alumina is transitioned to a α-Al2O3 crystal phase, in an oxidizing or neutral atmosphere.
The sintering may be performed by heating a rotary furnace at 1200 to 1650° C. using a gas burner, charging the dry-crushed product to the rotary tube furnace using a feeder for fuel supply, sintering the charged product while the charged product flows toward the gas burner due to the rotation of a tilted rotary tube furnace, transferring the sintered product to a tilted rotary cooler into which a cold air flows for cooling, and classifying the cooled product using a vibrating screen to obtain an alumina-based abrasive grain having a desired particle size distribution.
The method of manufacturing an alumina-based abrasive grain may further include performing dry crushing of the dried product between the step (f) and the step (g). The dry crushing of the dried product may be performed by dry ball milling, an inner wall of a ball mill for the dry ball milling may be lined with alumina, and a ball used for the dry ball milling may be formed of alumina.
In another aspect, the present disclosure provides an alumina-based abrasive grain manufactured by the method of manufacturing an alumina-based abrasive grain, which has a Vicker's hardness of 15 to 25 MPa, a fracture toughness of 2 to 10 MPa, and a bulk density of 3.70 to 3.92 g/cm3, and includes a corundum crystal having a size of 0.1 to 2 μm.
According to the present disclosure, an alumina-based abrasive grain which is highly dense, has a high hardness, and exhibits a high purity can be manufactured using boehmite powder and activated alumina powder.
In addition, according to the present disclosure, a sintered alumina-based abrasive grain is classified by particle size to have a particle size distribution according to nominal specifications (KS, ISO, FEPA, etc.).
The alumina-based abrasive grain manufactured according to the present disclosure has a Vicker's hardness of 15 to 25 MPa, a fracture toughness of 2 to 10 MPa, and a bulk density of 3.70 to 3.92 g/cm3, is highly dense, and exhibits a high hardness and a high purity.
In addition, in the alumina-based abrasive grain manufactured according to the present disclosure, corundum crystals having a size of 0.1 to 2 μm are densely linked.
In addition, according to the present disclosure, the alumina-based abrasive grain can be easily manufactured using solid-phase boehmite (AlOOH) powder and activated alumina, which include an aluminum (Al) component, can have a high reproducibility due to a simple process, and can be produced on a large scale.